1. Field of the Invention
The present invention relates to electronic systems and methods for controlling luminance of light sources, and particularly to a system and method for controlling luminance of an LED (Light Emitting Diode) lamp as an indicator.
2. Related Art
Recently indicators that use LEDs have been introduced in various electronic apparatuses. The electronic apparatuses employ the LED lamp as an indicator for indicating a particular working state. For example, when the LED lamp is lighted on, that means the electronic apparatus is powered on; alternatively, when the LED is black out, that means the electronic apparatus is shut down. The brightness of the LED lamp depends on the current flow through it. That is, the luminance value of the LED lamp is proportional to LED current. Therefore, correspondingly to a particular working state, it is necessary to provide a corresponding current control circuit to obtain a particular luminance value of the LED lamp. In other words, in order to indicate various working states, there are provided many LED lamps and the corresponding current control circuits as well as the working states. As result, the cost and complexity of manufacture associated with the electronic apparatuses are increased. Therefore, there is a need for a system and a method for effectively controlling a LED lamp to indicate different working states.
In a typical lighting system such as a backlight module used for a liquid crystal display (LCD), to control brightness thereof, a driver circuit increases or decreases a drive current supplied to an indicator lamp. Generally, the drive current is adjusted in relation to the ambient light environment and according to user preferences. A poorly lit environment usually requires less brightness, and thus a lower drive current, than a brightly lit environment. Different users may have different desired brightness levels. The brightness may be changed automatically in response to the environment and/or changed manually. However, such changes may rapidly reduce the useful operating lifetime of the lamp. Many control schemes like PWM (Pulse-Width Modulation) duty cycle controls have been implemented in order to control lamp lighting. PWM duty cycle control of the lamp luminance is accomplished by duty cycle control of the lamp on a time basis relative to a total time period. During the “on” time of the PWM signal, a higher frequency current supply is provided to the lamp.
U.S. Pat. No. 6,388,388 published on Dec. 27, 2000 provides a “Brightness control system and method for a backlight display device using backlight efficiency.” The brightness control system uses the efficiency of the backlight in order to achieve a desired brightness or luminance for the backlight display device. At each lamp temperature, the luminance is linearly proportional to a desired drive current for the backlight. By using the measured lamp temperature and known backlight efficiency to derive a desired lamp current and then generating the desired lamp current, the brightness control system can maintain the desired brightness throughout the dynamic range of operation of the backlight display device. The desired lamp current may be computed in a digital micro controller in response to a digitized lamp temperature measurement. This lamp current value is converted to an analog signal having a magnitude that creates the desired average lamp current. The analog signal is coupled to an IC (integrated circuit) inverter as a brightness command. Although the lamp current necessary to create the desired illumination is known, however, the lamp current is adjusted based on the temperature thereof which may be affected by the ambient, so that the lamp current may be inaccurate. Thus, there is a need for a system and method for effectively controlling luminance of an LED lamp.